Bake potatoes at 375° 1 hour, or until easily pierced with fork. While potatoes are baking, line strainer with cheesecloth and set over bowl. Add yogurt and set aside to drain about 15 minutes. Transfer to small bowl. Whisk in mustard and set aside. In large nonstick frying pan over medium-high heat, saute onions and garlic in oil until onions wilt, about 5 minutes. Add mushrooms, dill, thyme and nutmeg. saute until mushrooms are brown and fragrant, about 10 minutes. Remove from heat. Stir in yogurt mixture. Serve over potatoes.